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<li class="toctree-l3 current"><a class="current reference internal" href="#">永久只读缓存</a><ul>
<li class="toctree-l4"><a class="reference internal" href="#index-0">共享、只读的父映像缓存</a></li>
<li class="toctree-l4"><a class="reference internal" href="#id3">不可变对象缓存守护进程</a></li>
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<div id="dev-warning" class="admonition note">
  <p class="first admonition-title">Notice</p>
  <p class="last">This document is for a development version of Ceph.</p>
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    <a href="https://pad.ceph.com/p/Report_Documentation_Bugs">Report a Documentation Bug</a>
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  <section id="rbd">
<h1>RBD 永久只读缓存<a class="headerlink" href="#rbd" title="Permalink to this heading"></a></h1>
<section id="index-0">
<span id="id1"></span><h2>共享、只读的父映像缓存<a class="headerlink" href="#index-0" title="Permalink to this heading"></a></h2>
<p>从父映像 <a class="reference external" href="../rbd-snapshot/#layering">克隆的 RBD 映像</a> 只对父映像更改了很小一部分。例如，
在一个 VDI 应用案例中， VM 都是从同一个基础映像克隆来的，
最初的差异只有主机名和 IP 地址。在启动期间，所有这些 VM 都会读取同一父映像数据的一部分。如果我们有这个父映像的本地缓存，
在这个有缓存的主机上就可以提速读取操作。
还能减少客户端到集群的网络流量。RBD 缓存必须在 <code class="docutils literal notranslate"><span class="pre">ceph.conf</span></code> 里显式地启用。
<code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 守护进程负责把父映像的内容缓存在本地磁盘上，
以后再有那些数据的读取就从本地缓存中读取。</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>RBD 共享、只读的父映像缓存需要 Ceph Nautilus 或更高版本。</p>
</div>
<p class="ditaa">
<img src="../../_images/ditaa-eb4eb82f75f67072b54e9cee3d527a10b9d8ac94.png"/>
</p>
<section id="id2">
<h3>启用 RBD 的共享只读父映像缓存功能<a class="headerlink" href="#id2" title="Permalink to this heading"></a></h3>
<p>要启用 RBD 共享、只读的父映像缓存功能，需要把下列选项加进
<code class="docutils literal notranslate"><span class="pre">ceph.conf</span></code> 文件的 <code class="docutils literal notranslate"><span class="pre">[client]</span></code> <a class="reference external" href="../../rados/configuration/ceph-conf/#configuration-sections">段</a> 下：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">rbd</span> <span class="n">parent</span> <span class="n">cache</span> <span class="n">enabled</span> <span class="o">=</span> <span class="n">true</span>
<span class="n">rbd</span> <span class="n">plugins</span> <span class="o">=</span> <span class="n">parent_cache</span>
</pre></div>
</div>
</section>
</section>
<section id="id3">
<h2>不可变对象缓存守护进程<a class="headerlink" href="#id3" title="Permalink to this heading"></a></h2>
<section id="id4">
<h3>介绍及常规选项<a class="headerlink" href="#id4" title="Permalink to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 守护进程负责把父映像内容缓存进本地的缓存目录里。
要想提高性能，建议用 SSD 作为底层存储器。</p>
<p>这个守护进程的关键组件有：</p>
<ol class="arabic simple">
<li><p><strong>基于 IPC 的域套接字:</strong> 这个守护进程在启动时会监听本地域套接字，
并等待 librbd 客户端连接。</p></li>
<li><p><strong>基于 LRU 的晋级、降级策略:</strong> 这个守护进程会在内存里维护各个缓存文件的命中统计信息。
如果缓存容量达到了配置的阈值，它就会降级冷缓存。</p></li>
<li><p><strong>基于文件的缓存库:</strong> 这个守护进程维护着一个简单的基于文件的缓存库。需要晋级时，
会从 RADOS 集群取回 RADOS 对象、并存储在本地的缓存目录里。</p></li>
</ol>
<p>打开各个克隆的 rbd 映像时， <code class="docutils literal notranslate"><span class="pre">librbd</span></code> 会尝试通过它的 Unix 域套接字连接到缓存守护进程。成功连接后， <code class="docutils literal notranslate"><span class="pre">librbd</span></code> 会和那个守护进程协调后续的读取操作。
如果有没缓存的读取，缓存守护进程会把那个 RADOS 对象晋级到本地缓存目录，
这样下次读取这个对象时就可以从缓存读取。缓存守护进程维护着简单的 LRU 统计信息，
以便容量不足时可以按需逐出冷缓存文件。</p>
<p>这里是一些重要的缓存配置选项：</p>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_sock</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>域套接字路径， librbd 客户端和
ceph-immutable-object-cache 守护进程之间通讯用的。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">/var/run/ceph/immutable_object_cache_sock</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_path</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存的数据目录。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>String</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">/tmp/ceph_immutable_object_cache</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_max_size</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变缓存的最大尺寸。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Size</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">1G</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_watermark</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>缓存的最大量（最高水位），数值在 (0, 1) 二者之间。如果缓存的尺寸达到了这个阈值，守护进程就根据 LRU 统计信息开始删除冷缓存。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Float</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">0.9</span></code></p>
</dd>
</dl>
<p>可选的 <code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 软件包里包含这个守护进程。</p>
<div class="admonition important">
<p class="admonition-title">Important</p>
<p><code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 守护进程要能连接 RADOS 集群。</p>
</div>
</section>
<section id="id5">
<h3>如何运行不可变对象缓存守护进程<a class="headerlink" href="#id5" title="Permalink to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 守护进程应该用一个唯一的 Ceph 用户 ID 。
要 <a class="reference external" href="../../rados/operations/user-management#add-a-user">新建一个 Ceph 用户</a> ，用 <code class="docutils literal notranslate"><span class="pre">ceph</span></code> 命令加上 <code class="docutils literal notranslate"><span class="pre">auth</span> <span class="pre">get-or-create</span></code> 子命令、
用户名、监视器能力、和 OSD 能力：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ceph</span> <span class="n">auth</span> <span class="n">get</span><span class="o">-</span><span class="ow">or</span><span class="o">-</span><span class="n">create</span> <span class="n">client</span><span class="o">.</span><span class="n">ceph</span><span class="o">-</span><span class="n">immutable</span><span class="o">-</span><span class="nb">object</span><span class="o">-</span><span class="n">cache</span><span class="o">.</span><span class="p">{</span><span class="n">unique</span> <span class="nb">id</span><span class="p">}</span> <span class="n">mon</span> <span class="s1">&#39;allow r&#39;</span> <span class="n">osd</span> <span class="s1">&#39;profile rbd-read-only&#39;</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 守护进程可以用 <code class="docutils literal notranslate"><span class="pre">systemd</span></code> 管理，
指定用户 ID 作为守护进程例程：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">systemctl</span> <span class="n">enable</span> <span class="n">ceph</span><span class="o">-</span><span class="n">immutable</span><span class="o">-</span><span class="nb">object</span><span class="o">-</span><span class="n">cache</span><span class="nd">@ceph</span><span class="o">-</span><span class="n">immutable</span><span class="o">-</span><span class="nb">object</span><span class="o">-</span><span class="n">cache</span><span class="o">.</span><span class="p">{</span><span class="n">unique</span> <span class="nb">id</span><span class="p">}</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 也能在前台运行，用 <code class="docutils literal notranslate"><span class="pre">ceph-immutable-object-cache</span></code> 命令:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">ceph</span><span class="o">-</span><span class="n">immutable</span><span class="o">-</span><span class="nb">object</span><span class="o">-</span><span class="n">cache</span> <span class="o">-</span><span class="n">f</span> <span class="o">--</span><span class="n">log</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="p">{</span><span class="n">log_path</span><span class="p">}</span>
</pre></div>
</div>
</section>
<section id="qos">
<h3>QoS 选项<a class="headerlink" href="#qos" title="Permalink to this heading"></a></h3>
<p>不可变对象缓存支持节流，由下列选项控制：</p>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_schedule_tick_min</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存的最小时间片。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Milliseconds</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">50</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_iops_limit</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 操作期望的每秒限额。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Unsigned Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">0</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_iops_burst</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 操作允许的爆发量。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Unsigned Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">0</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_iops_burst_seconds</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 操作允许的爆发时长。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Seconds</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">1</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_bps_limit</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 期望的每秒的字节数限额。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Unsigned Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">0</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_bps_burst</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 允许的字节数爆发量限额。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Unsigned Integer</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">0</span></code></p>
</dd>
</dl>
<p><code class="docutils literal notranslate"><span class="pre">immutable_object_cache_qos_bps_burst_seconds</span></code></p>
<dl class="field-list simple">
<dt class="field-odd">描述<span class="colon">:</span></dt>
<dd class="field-odd"><p>不可变对象缓存 IO 允许的爆发秒数。</p>
</dd>
<dt class="field-even">类型<span class="colon">:</span></dt>
<dd class="field-even"><p>Seconds</p>
</dd>
<dt class="field-odd">是否必需<span class="colon">:</span></dt>
<dd class="field-odd"><p>No</p>
</dd>
<dt class="field-even">默认值<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">1</span></code></p>
</dd>
</dl>
</section>
</section>
</section>



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